1. Field of the Invention
This invention relates generally to manholes and more particularly, to manhole safety steps, plugs for use therewith and a novel method of constructing manhole casings with the plugs preset therein.
2. Description of the Prior Art
In most road, sewer or other similar construction projects, manholes are installed to provide access to underground areas to allow for the installation, repair or inspection of electrical and/or telephone installations, storm drainage and sanitary sewers and appurtenances and other similar installations, without the need to tear up the road or sidewalk to gain the necessary access.
A typical manhole may be of round or square dimensions with a diameter of between about 42" to 72" and it may extend, for example, from the surface of the ground downwardly to an underground area or interconnecting tunnel which contains electrical or telephone cables, to a sewer conduit or a pumping station. The manhole, usually consists of one or more riser sections, each of which consists of a casing and a top section, and the manhole is set in the ground and earth is backfilled about the casing to secure the casing in the desired position. A removable top or cover may be placed over the casing and a means is provided for access to the interior of the manhole. The casings may be reinforced, if desired.
The casings for the manholes are usually manufactured of concrete of for example 4000 to 6000 psi and may be of any desired length, usually in the range of three feet to six feet. These dimensions may vary depending upon the specifications required for each project. Any number of sections may be used to produce the manhole of the desired length.
For ease of reference, the description of this invention will be made in conjunction with a single riser section. However, it is to be understood that any number of riser sections may be used in the production of a manhole of the desired height. The risers are vertically aligned with each other and secured in any method well-known and familiar to those in this art. For a description of precast concrete manhole sections, reference is made to the Annual Book of ASTM Standards, C478-73, American Society For Testing and Materials (1973).
The method of manufacturing the manhole casing may also vary but usually includes the steps of pouring the concrete into a form, compacting and curing the concrete. This may be done either at a prefabrication site or at the job site. Once the manhole casing is positioned, earth is backfilled about the manhole casing to secure it in the desired location.
In order to allow easy access to the bottom of the manhole, some form of steps must be provided along the interior vertical surface of the casing to allow the workman to climb up or down the manhole. Generally, the steps are either in the form of a portable or removable ladder or steps embedded in the side wall of the manhole casing.
Portable ladders are generally not used in the trade because of the obvious drawback that the workman must carry the ladder with him to each manhole. Also, there is a serious danger that the ladder may dislodge itself during use thus causing the workman to fall.
Manholes with steps provided in the wall of the casing thus have represented an improvement over the use of portable ladders. The steps are permanently and safely secured in the concrete wall of the manhole casing. These steps are generally U-shaped and are placed horizontal with the free ends of each arm embedded in the concrete wall of the manhole casing. The base portion of each step is dropped slightly from the level of the arms to provide an additional safety feature.
In order to secure the arms of the steps in the concrete wall of the casing, various different methods have been suggested by the prior art.
One of these methods of the prior art is to secure the step in place with the form and then to pour the concrete and cure it with the arms of the steps in place. This method is particularly cumbersome in that a complicated form is required and it is difficult to remove the form from the concrete with the steps in place.
More recently, cored holes are formed in the wall of the manhole casing and the arms of the steps are then inserted into the cored holes and grouted into place. This may be done, for example, by drilling holes into the concrete manhole casing and securing the arms of the steps into the drilled hole.
In order to provide a secure fit, it is necessary for these holes to be drilled to exact tolerances. Also, this process is very time consuming and adds considerable cost to the manufacture of the manhole casing.
A further disadvantage with the prior art is that the plugs employed therein do not provide a secure arrangement and the arms of the step may pull loose thus creating a dangerous situation for the workman.
Some of these plugs may well hold the arms of the step in place under normal use. However, they fail when extraordinary pressure is applied to the step. For example, if a workman misses a step with his foot and falls to the next step, a substantially larger amount of pressure is applied to the second or lower step. The step must be able to withstand this sudden and extraordinary pressure applied to it without its arms being pulled from the wall.
One example of the method of securing the plug and the step of the prior art is shown in Canadian Pat. No. 676,968 granted to Marino on Dec. 31, 1963. This patent shows the use of a locking means such as a loose pin to secure the step to an aluminum sleeve which has been grouted into the wall. A serious drawback with this invention is that the locking means (that is, the pin) may be removed or loosened by any person thus creating a potentially dangerous situation for the workman.
A second plug arrangement is shown in Canadian Pat. No. 956,153 granted to Zenhausern on Oct. 15, 1974 which shows a plug inserted into the interior of the wall with the arms of the step inserted into the plug. This arrangement requires the drilling of holes to exact tolerances to insure a good grip for the plug.